The present invention relates to a receipt circuit included in a TDMA (Time Division Multiple Access) radio apparatus and, more particularly, to a receipt circuit for a TDMA radio apparatus of the type using a SAW (Surface Acoustic Wave) filter or a ceramic filter as band-pass filter for an IF (Intermediate Frequency) signal.
In one of conventional receipt circuits for a TDMA radio apparatus, a TDMA signal coming in through an antenna is input to an RF (Radio Frequency) amplifier via an input filter. The TDMA signal is amplified by the RF amplifier and then transformed to an IF signal by a mixer. An IF band-pass filter implemented by a SAW filter or a ceramic filter filters out needless signals and interference signals contained in the IF signal. A limiter amplifier amplifies the filtered IF signal while detecting its receipt electric field level. The resulting output of the limiter amplifier is fed to a demodulator.
The above receipt circuit apparatus has the following problems left unsolved. When a TDMA IF signal of high level is input to the SAW filter or the ceramic filter, the trailing edge of the signal is blunted at each time slot due to the influence of a group delay characteristic particular to the filter. Specifically, although the trailing edge is originally sharp, it is blunted by the SAW filter or the ceramic filter with the result that a component belonging to a preceding time slot remains in a guard time between the above time slot and the following time slot. It follows that when a TDMA signal of low level occurs immediately after the TDMA signal of high level, the receipt sensitivity of the former is lowered by the influence of the residual component of the latter. Moreover, if the residual component in the guard time is of high level, a carrier sensing function determines, even when an electric field is absent in the next time slot, that an input electric field is present. This practically disables the radio apparatus from communicating with another radio apparatus in both of a transmission time slot and a receipt time slot.
It is therefore an object of the present invention to provide a receipt circuit for a TDMA radio apparatus capable of obviating, when a TDMA IF signal of high level is input to a SAW filter or a ceramic filter, the fall of receipt sensitivity and the inability to communicate ascribable to the influence of the group delay characteristic particular to the filter.
A receipt circuit for a TDMA radio apparatus of the present invention includes a band-pass filter for an IF signal and implemented by either one of a SAW filter and a ceramic filter, and a removing circuit for removing a residual signal component remaining in a preselected guard time between consecutive time slots of a TDMA IF signal passed through the band-pass filter.